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The Greatest Volcanic Ash Falls in the Phanerozoic: Trans-Atlantic Relations of the Ordovician Millbrig and Kinnekulle K-Bentonites Stig M

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The Greatest Volcanic Ash Falls in the Phanerozoic: Trans-Atlantic Relations of the Ordovician Millbrig and Kinnekulle K-Bentonites Stig M The Sedimentary Record The Greatest Volcanic Ash Falls in the Phanerozoic: Trans-Atlantic Relations of the Ordovician Millbrig and Kinnekulle K-Bentonites Stig M. Bergström, Department of Geological Sciences, The Ohio State University, Columbus, OH 4321; [email protected] Warren D. Huff, Department of Geology, University of Cincinnati, Cincinnati, OH 45221; [email protected] Matthew R. Saltzman, Department of Geological Sciences, The Ohio State University, Columbus, OH 43210; [email protected] Dennis R. Kolata, Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, IL 61820; [email protected] Stephen A. Leslie, Department of Earth Science, University of Arkansas at Little Rock, Little Rock, AR 72204; [email protected] Figure 1: Outcrop of the Millbrig K-bentonite ABSTRACT along U.S. Highway 68 at entrance to Beds of altered volcanic ash (K-bentonites) are often widespread geographically, but they Shakerstown, Kentucky. The ash bed is about generally occur sporadically in the lower Paleozoic geologic record. Most occurrences 70 cm thick at this section. Head of top have been recorded from North America, northern Europe, and Argentina.These ash hammer marks the base of the bed. beds are in most cases less than 20 cm thick but three exceptional beds in the Ordovician, the Deicke and Millbrig in North America and the Kinnekulle in northern ticularly thick beds, the Deicke and the Europe, reach thicknesses of 1-2 m or more and can be traced over millions of km2. Each Millbrig K-bentonites (Figure 1), which have has an estimated dense rock equivalent volume of around 1000 km3 or more, and they been traced over most of the eastern and cen- represent the largest Phanerozoic volcanic ash falls recorded.Various lines of evidence, tral part of the continent (Huff and Kolata, particularly biostratigraphic and chemostratigraphic, indicate no significant age difference 1990; Kolata et al., 1996; Fig. 2). In northern between the Millbrig and Kinnekulle beds but whether or not these originated from the Europe, there is one especially prominent ash same gigantic eruption(s) is still speculative.At any rate, these ultraplinian eruptions bed, the Kinnekulle K-bentonite, which has appear to have been derived from a volcanic arc or microplate subduction zone along the been identified across Scandinavia and the eastern side of Laurentia. In view of their enormous size and continent-wide distribution, Baltic States and locally in Great Britain it is surprising that these ash beds are not directly associated with any notable faunal (Bergström et al., 1995). The Millbrig, extinction event or any significant lithologic change in the sedimentary record. Deicke, and the Kinnekulle are locally 1-2 m or more thick and before compaction, they apparently represented ash layers with an orig- INTRODUCTION beds are apparently rare but more than 60 inal thickness of at least four times that figure. In terms of destruction of land and life, vol- such beds have been recorded from the The Millbrig covers an estimated area of at canic eruptions surely rank as some of our Ordovician of North America and more than least 2.2x106 km2 in North America and the planet’s most severe catastrophes. In the time 150 from Baltoscandia (Kolata et al., 1996). Kinnekulle at least 6.9x105 km2 in northwest- interval of human history, some such erup- Although Silurian (Bergström et al., 1998; ern Europe. In terms of dense rock equiva- tions have been especially devastating, perhaps Huff et al., 2000) and Devonian (Ver lent, the silicic magma is estimated to have the most well-known one being the Vesuvius Straeten, 2004) K-bentonites have been been 1509 km3 for the Millbrig and 972 km3 eruption in 79AD. As vividly reconstructed in recorded from several regions, such ash beds for the Kinnekulle (Huff et al., 1996). These Robert Harris’ (2003) fascinating novel appear to be less common in those systems values do not include the vast quantity of vol- Pompeii, this disastrous eruption destroyed in than in the Ordovician. Interestingly, concen- canic ash presumably deposited in the Iapetus a couple of days the towns of Pompeii, trations of K-bentonites in the geologic record Ocean between Baltica and Laurentia and Herculaneum, and Stabiae and killed virtually appear to reflect collisional orogenic episodes, subsequently subducted. The truly gigantic all of their inhabitants. many of the Ordovician and Devonian ones size of these ash falls is indicated by the fact The presence of volcanic ash beds in many being coeval with the Taconic and Acadian that the dense rock equivalent of the 1980 geologic successions provides evidence of simi- orogenies, respectively. Mt. St. Helens eruption is estimated at 0.2 lar, but in many cases far greater, volcanic ash The thickest and most widespread km3 (Huff et al., 1992). These enormous ash eruptions in the Earth’s pre-human history. Paleozoic K-bentonites occur in the Middle falls are the largest recorded in the Earth’s The present account centers on beds of altered Ordovician Mohawkian Stage in North Phanerozoic history. volcanic ash, known as K-bentonites, in the America (Kolata et al., 1996) and in the Many aspects of the distribution, mineralo- lower Paleozoic of North America and north- Keilan Stage in Baltoscandia (Bergström et al., gy, bio- and chemostratigraphy, isotope geolo- ern Europe. In the Cambrian K-bentonite 1995). In North America, there are two par- gy, and the eventstratigraphic and paleogeo- 4 | December 2004 The Sedimentary Record increase from Estonia to southwestern Sweden and southern Norway which indicates that the source of the ash was located southwest of Scandinavia. Both in North America and Baltoscandia, these thickness trends are associ- ated with a general increase in grain size toward the source area. According to Huff et al. (1996), the grain size and distribution patterns indicate that the ash originated from plinian and co-ignimbrite eruptions and that the pyro- clastic material was widely distributed by equa- torial stratospheric and tropospheric winds. Plotted on widely used paleogeographic recon- structions, the distribution patterns of the Millbrig and Kinnekulle beds (Figure 3) are not in conflict with the idea that they had a com- mon source. Although interesting, these distri- bution patterns do not provide any conclusive Figure 2: Stratigraphic cross section from Minnesota to Alabama-Georgia showing the stratigraphic evidence of the direct source relationships of position of the Millbrig and Deicke K-bentonites. (Modified from Huff and Kolata, 1990.) these ash beds. graphic significance of these ash beds have DISTRIBUTION BIOSTRATIGRAPHY been investigated, particularly during the last As noted above, the Millbrig has been identi- Of crucial importance for clarifying the rela- two decades (see, e.g., Kolata et al., 1996). fied over much of eastern and central North tionships between the Millbrig and One interesting problem that has attracted America, from the Mississippi region and Kinnekulle beds is obviously to be able to special attention and some recent controversy Oklahoma to the Appalachians (Kolata et al., demonstrate that they are of the same age. is whether the Millbrig and the Kinnekulle 1996; Figure 2). A maximum thickness map Possible methods for this include biostratigra- beds are equivalent and may represent the produced by Huff et al. (1996; Figure 3) shows phy and radiometric dating but both these same colossal eruption. If that was the case, a gradual increase in thickness from a few cm approaches have their special problems. the combined dense rock equivalent of these in the Mississippi Valley to 1-2 m in the south- Fortunately, both the Millbrig and the beds would be on the order of 2500 km3. Such eastern portion of the continent suggesting that Kinnekulle beds occur in successions contain- an eruption ought to have had profound cli- the source volcano(es) of the ash was located ing biostratigraphically highly diagnostic fos- matic and biological effects on a global scale. off the southeastern part of the continent (pres- sils such as graptolites and conodonts. In In the present contribution, we summarize ent-day coordinates). A similar map of thick- North America, e.g. in the section at various types of data that bear on the consan- ness trends of the Kinnekulle bed (Huff et al., Strasburg, Virginia (Finney et al., 1996), the guinity of these extraordinary ash beds. 1996; Figure 3) shows a marked thickness Millbrig is associated with graptolites of the Iapetus Ocean Figure 3: Sketch map of central and eastern USA and Baltoscandia showing study localities (dots) and thickness trends (cm) of the K-Bentonite Source? Millbrig and Kinnekulle 20010080 6040 K-bentonites, respectively. 3 10 (Modified after Huff et 20 40 20 10 al., 1996.) The contours 100 140 are estimated based on 30˚ S 60 the thicknesses recorded at sites marked by dots. Contour estimates far away from data points 0 300 Km are less well constrained. December 2004 | 5 The Sedimentary Record NORTH AMERICA BALTOSCANDIA and in conflict with all biostratigaphic evi- RANGES CONODONT N. AM. K-BENTONITES dence. As shown by Goldman and Bergström K-BENTONITES ZONE OF KEY & GRAPTO- & 13 13 GRAPTO- (1997), the D. complanatus Zone corresponds LITE Z. ZONE STAGE C EXCURSION C EXCURSION LITE Z. MIDCONT. ATL. CONOD. LITES GRAPTO- to part of the Richmondian Stage of the G. Upper Ordovician in North America, an pygmaeus interval several stages above the position of Am. the Millbrig in the upper Middle Ordovician EDENIAN STAGE B. superbus confluens C. Chatfieldian Stage. spiniferus Am. superbus Am. 13 δ C carb. 13 δ C carb. CHEMOSTRATIGRAPHY 0 1 2 3 0 1 2 O. Extensive research in recent years (see, e.g., ruede- RAKVERIAN manni OAN- Ludvigson et al., 2004; Saltzman et al., 2003) DUAN 13 P.
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